The present invention relates to an exposure and development control apparatus for an electrostatic copying machine or the like.
In an electrostatic copying machine, the illumination intensity of an original document and a developing bias voltage applied to photoconductive drum on which is formed an electrostatic image of the document must be set to proper levels in order to produce copies of good quality. Manual adjustment of these parameters based on operator judgement almost always results in copies of poor quality, especially where the operator has minimal experience.
Generally, the exposure intensity and bias voltage are dependent on the contrast and maximum and minimum densities of the original document. The proper illumination intensity is a function of the density of the background areas of the document, or the minimum density. The bias voltage, however, depends on both the density of the background areas and the contrast between the maximum (dark area) and minimum densities. Some documents have low density both in the background and dark areas, and thereby low density and contrast. Other documents have colored or gray background areas with high density dark areas. These documents are also low in contrast although high in density. Adjustment of the development bias voltage as a function of only the background density would result in a washed-out or low density copy in all cases of low contrast. For this reason, it is desirable to control the bias voltage as a function of both background density and contrast in combination with control of the illumination intensity. Apparatus heretofore proposed to accomplish this function have not demonstrated satisfactory performance.